U.S. Pat. Nos. 4,918,128 and 5,190,818 provide pressure sensitive adhesives/cohesives that are revolutionary in their utility when utilized with mailers or like type documents or business forms. These patented adhesives have allowed the practical implementation in the mailer industry of mailers which may be quickly and effectively produced and sealed without the drawbacks associated with conventional heat or moisture activated adhesives.
While the adhesives in the above-identified patents are extremely effective, it is desirable to be able to produce an adhesive that has even better properties. While the adhesives in the above-identified patents are not tacky after the application to paper, and do not adhere/seal unless a high level of pressure is applied (which prevents the adhesive from blocking during the printing, which would slow down the processing due to paper jams or the like), sometimes the adhesive action may be hampered by contamination of the printing surface and adhesive with silicone lubricants used in conventional printers (particularly laser printers).
The adhesive according to the present invention, when coated on paper or like substrates used in mailer-type business forms, or the like, has the same advantageous properties as the adhesives in the above-identified patents and also has enhanced functionality when contaminated by silicone lubricants.
Like the preferred embodiments in the above-identified patents, the cohesive according to the present invention contains a natural rubber graft, an appropriate acrylate monomer or monomers such as methyl methacrylate, or cyclohexyl methacrylate, or benzyl methacrylate, or isobornyl methacrylate or trimethyl cyclohexyl methacrylate or isobornyl acrylate), and finely divided hard particles such as silica. When cyclohexyl methacrylate is employed it is typically present in an amount of about 0.5-1.5 weight %, more usually about 1 weight % based in the weight of the modified latex. Methyl methacrylate and polymethylmethacrylate when employed are typically present in an amount of about 10-18 weight %, more usually about 15 weight %.
The cohesive according to the present invention also may include starch, acrylic acid and/or 4-acetoxystyrene, and optionally ethyl hexyl acrylate.
According to the present invention a pressure sensitive adhesive/cohesive is provided comprising the following components: 100 parts by weight natural rubber; about 5-35%, by weight of the rubber, acrylate monomer; about 0.5-8% by weight of the rubber acrylic acid and/or about 0-10%, typically 1-10%, by weight of rubber, 4-acetoxystyrene, and 1-50%, by weight of the rubber, a finely divided hard particulate material having substantially no thermoplasticity.
The composition may also comprise advantageously about 0-10%, for example 1-10%, by weight of rubber, 4-acetoxystyrene. The composition may further comprise about 0-20%, by weight of the rubber, ethyl hexyl acrylate, 0-50%, by weight of the rubber, starch; and 0-40%, by weight of the rubber, carboxylated styrene-butadiene latex, or carboxylated polychloroprene latex, or vinyl-pyridine styrene-butadiene latex or pre-crosslinked natural rubber latex, or a tackifier, or combinations thereof.
In the case of 4-acetoxystyrene as a comonomer in the graft/block of the rubber, grafted rubber has built-in antioxidant as a result of hydrolysis of acetoxy group which provides the cohesive antioxidant property helping its shelf-life as a coated cohesive by acting as a free radical scavenger.
The pressure sensitive adhesive/cohesive according to the present invention is typically coated on a piece of paper. More typically, since the material according to the invention is best as a cohesive, it is coated on two portions of a piece of paper, the cohesive portions being in contact with each other as a result of folding the paper (such as by forming a mailer), or bringing a like sheet in contact therewith, and pressure sealed together (such as by running through conventional pressure sealing equipment, such as xe2x80x9cSpeedisealer(copyright)xe2x80x9d equipment available from Moore USA of Lake Forest, Ill.), so as to cause paper fiber tear if attempted to be pulled apart.
In the following discussion, percentages are by weight unless otherwise stated. Preferably, the adhesive/cohesive comprises at least 1% ethyl hexyl acrylate, and at least 1% starch, e.g. about 10-30% starch (such as about 20% starch) having an average particle size of about 5-25 microns. The hard particulate material preferably comprises silica gel, e.g. about 10-30% (such as about 20%), the silica gel having an average particle size of about 0.2-20 microns (e.g. about 0.3-0.4 microns), or a fumed silica (e.g. 0.1-0.3 microns). There also may be at least 5% carboxylated styrene-butadiene latex, or carboxylated polychloroprene latex, or vinyl-pyridine styrene-butadiene latex, or pre-crosslinked natural rubber latex, or a styrene-acrylate-acrylonitrile latex, or a tackifier, or combinations thereof.
The acrylate monomer preferably comprises methyl methacrylate, e.g. about 5-30%, or 0.5-8% of 4-acetoxystyrene, cyclohexyl methacrylate, benzyl methacrylate, trimethyl cyclohexyl methacrylate or isobornyl methacrylate. The acrylic acid typically is present between about 0.5-4%, and the 4-acetoxystyrene between about 1-3%. The modified natural rubber latex is preferably electrosterically stabilized natural rubber graft and block terpolymer.
Generally, the acrylate monomer is selected from cyclohexyl methyl methacrylate, methyl methacryate and mixtures of the two. Trimethyl cyclohexyl methyl methacrylate, methyl methacryate and mixtures thereof may also be used. In both instances, the natural rubber may be electrostatically stabilized latex of natural rubber graft/block terpolymer.
In the present case, the term xe2x80x9celectrostericallyxe2x80x9d is used when there are ionizable/hydrolyzable groups grafted onto the natural rubber, e.g. acrylic acid or 4-acetoxystyrene. These ionizable groups impart a negative charge to the latex particles which stabilizes the emulsion. The term xe2x80x9celectrostaticallyxe2x80x9d is used when non-ionizable/hydrolyzable groups grafted onto the natural rubber and, hence, the only charge on the latex particles arises from an electric double layer.
According to a further embodiment, the cohesive may be manufactured by synthesizing a modified natural rubber latex through reaction of 1,4 cis polyisoprene (natural rubber) with two or more acrylate monomers, followed by further reactions and processing. Typically, the modification carried out using cyclohexyl methacrylate (CHMA) (approx. 1 to about 5% by weight) and/or methyl methacrylate (MMA) (approx. 10-18% by weight).
For purposes of discussion, the synthesis of the modified natural rubber latex graft/block terpolymer with CHMA/MMA will be described below. However, it will understood that the invention is not limited to modified natural rubber latexes using those two acrylate monomers. Examples of other acrylates which may be used are benzyl methacrylate and/or isobornyl methacrylate and/or trimethyl cyclohexyl methacrylate and/or isobornyl acrylate.
A graft/block terpolymer of natural rubber latex with CHMA/MMA may be synthesized by conducting a seeded emulsion polymerization of natural rubber latex (for example about 100 parts by weight) with CHMA (for example about 5 parts by weight), by initiating a graft polymerization by a redox couple under a nitrogen atmosphere in a reaction vessel at 35xc2x0 C. After CHMA starts to polymerize and starts to be consumed, MMA (about 11 parts by weight) is added to the reaction mixture by initiating a reaction with the redox couple initiator system. MMA polymerizes along with the growing polymer CHMA chains creating partial blocks of poly CHMA poly MMA grafted onto the 1,4-cis-polyisoprene.
This results in a modified natural rubber latex molecular architecture which is a graft/block terpolymer latex. Some polymethylmethacrylate (PMMA) is also formed during synthesis of the modified natural rubber latex because surfactant concentration is far above the cmc (critical micelle concentration) which promotes the formation of some emulsion polymerized MMA in the micelles. This is about 0.2% -0.5 wt % emulsion polymerized PMMA in the modified natural rubber colloid. Reaction of MMA and/or CHMA is believed to be accompanied by some chemical crosslinking to form a graft block terpolymer of 1,4 cis polyisoprene (natural rubber) with CHMA and PMMA.
The modified natural rubber latex is then blended with a styrene-butyl acrylate latex, typically Acrygen 41135 latex having a narrow particle size distribution (average particle size 400 nm), a surface tension of 49 dyne/cm, and a Brookfield viscosity (#2 @6 rpm) of 1500 cps. Acrygen 41135 latex possesses excellent mechanical stability, tensile strength 510 psi, and elongation of 700%, and is compatible with the modified natural rubber latex, functions as binder, has mechanical stability and acts as a colloidal stability enhancer. Chemical crosslinking between Acrygen 41135 and the modified natural rubber latex is not believed to occur.
A sulfated fatty acid, typically Modical S, is then added to the formulation as a mechanical and chemical stability enhancer. This is followed by blending the mixture to form a colloidally stable cohesive formulation.
Styrene present in the cohesive is from the Acrygen 41135 latex which is a alternating copolymer latex of styrene and butyl acrylate with a narrow particle size distribution. Styrene is not a part of the modified natural rubber latex employed as starting material.
In a yet further embodiment, the cohesive of the invention may be synthesized as follows.
Modified latex (a graft block terpolymer of 1,4 cis polyisoprene (natural rubber-synthesized as described above) is blended with Acrygen 41135 latex (Omnova Solutions) in a blender. Modical S (Henkel corporation), a sulfated fatty acid, is added to the formulation as a mechanical and chemical stability enhancer, followed by blending of all of the components to form a colloidally stable mixture. A silica hydrogel slurry, typically Syloid W-300 (Grace-Davison corporation), is added to the colloidal mixture and stirred. Syloid W-300 has an average particle size of 5 xcexcm and a pore volume of 1.2 cc/gm. The silica hydrogel also modifies the modulus of the cohesive composite and helps to absorb the silicone lubricant.
An acetylenic diol nonionic surfactant with a mid HLB (hydrophilic lypophilic balance) range=+13, typically Surfynol GA (Air Products and Chemicals), is added to the colloidal mixture and the mixture is stirred. The Surfynol GA functions as a pigment disperser and helps the colloidal stability of the formulation.
A silicone based defoamer, for example SN-381 (San Nopco) is added to the mixture and the mixture is stirred. This is followed by addition of a sodium polyacrylate thickener with a very high pseudoplastic index, typically Alocgum 296W (Alco Chemical), and the mixture is stirred until the thickener is dispersed. An ammoniacal solution of yellow dye, typically FDC No.5 (B.F. Goodrich - trisodium salt of 4,5-dihydro-5-oxo-1 (4-sulfophenyl)-4-[4-sulfophenylazo]-1H-pyrrazole-3-carboxylic acid) is added and the mixture is stirred.
An antimicrobial agent, typically Dowisil 75, active ingredient 1-(3-chloroallyl)-3,5,7-triazaadamantane chloride (Dow Chemical), is then added to the formulation and the mixture is stirred, followed by addition of an antioxidant, typically Tinox # 22MB - 2,2xe2x80x2-methylene bis (4-methyl-6-tert-butyl-phenol (Technical Solutions Inc.) in aqueous dispersion, to the colloidal mixture and the mixture is stirred. This antioxidant controls high shear mechanical degradation of polymer chains of the modified natural rubber latex (graft block terpolymer of 1,4 cis polyisoprene (natural rubber) with CHMA and PMMA), which occurs when the formulation is applied under high shear on the coating press. The antioxidant also helps to prevent the crosslinking of the modified natural rubber latex.
According to another aspect of the present invention, a method of making a mailer type business form is provided. The method comprises: (a) applying cohesive as cooperating patterns to a sheet of paper so that when the sheet is folded, or brought into contact with a like sheet, the patterns move into contact with each other. The cohesive comprises a pressure sensitive cohesive including: 100 parts by weight natural rubber; about 5-35% by weight of the rubber acrylate monomer; about 0-8% (preferably about 0.5-8%) by weight of the rubber acrylic acid and/or about 0-10% (preferably about 1-10%) by weight 4-acetoxystyrene; about 0-20% by weight of the rubber ethyl hexyl acrylate; 1-50% by weight of the rubber a finely divided hard particulate material having substantially no thermoplasticity; 0-50% by weight of the rubber starch; and 0-40% by weight of the rubber carboxylated styrene-butadiene latex, styrene-acrylate-acrylonitrile latex, or carboxylated polychloroprene latex, or vinyl-pyridine styrene-butadiene latex, or pre-crosslinked natural rubber latex, or a tackifier, or combinations thereof. When the film is formed on the surface paper, there is a compatibilizing effect between the Acrygen 41135 a styrene-acrylate latex which helps to change the surface characteristics of the film. Acrygen 41135 has a glass transition temperature (Tg) of 15xc2x0 C.
The method may comprise the further steps of: (b) folding the paper to move the cohesive patterns into contact with each other, and (c) applying a sealing pressure of at least about 100 lbs/lineal inch (e.g. about 200 lbs/lineal inch) to the patterns to seal the cohesive together so as to cause fiber tear if attempted to be pulled apart.
The present invention provides a pressure sensitive cohesive/adhesive which exhibits excellent adhesive bonding, cohesive bonding, resistance to heat, blocking resistance, resistance to abrasion, non-tackiness, good creep properties, and substantially no loss in adhesion/cohesion upon exposure to heat and polysiloxane based lubricants. Further aspects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.
The general invention as described above will now be set forth with respect to some specific examples.